The present invention pertains to a method and apparatus for drying a sample stream of gas and more particularly to a new and useful method and apparatus for removing the condensate from a gas stream while minimally interfering with the flow of the sample stream.
Presently, gas analyzers have instantaneous measurement of component concentration capabilities. However, in some applications, such as the measurement of component concentration in automobile exhaust gas, the instantaneous analysis capability is defeated by its moisture content. Although the analyzer can respond instantaneously to concentration changes, the moisture in the sample stream of exhaust gas must be removed in order to permit an accurate analysis of the component concentration. Prior art gas dryers have a significant deficiency in this area. Referring to FIG. 1, it can be seen that the gas concentration at 5 is significantly different from the concentration at 6 corresponding to the inlet and outlet of a volume 9 used for removing moisture from a sample stream of gas. As illustrated in FIG. 1, prior art condensate removal systems have an intake 7 and exhaust 8 with a large volume 9 therebetween. This large volume causes a mixing or integrating between the sample arriving at intake 7 and the sample contained in the volume used in cooling the sample gas. As a result, the concentration at exhaust 8 will not immediately represent the concentration at intake 7 but will initially represent the concentration in volume 9, then represent the mixture of the concentration in volume 9 plus the concentration at intake 7, and finally represent the concentration at intake 7. This process can best be seen through a comparison of curve 2A and 2B of FIG. 2. Curve 2A represents the concentration of a gas to be measured at intake 7 with respect to time. Curve 2B represents the concentration of the same gas at exhaust 8 with respect to time for the same time period. Since both concentrations are measured on the same time scale, it can easily be seen that a time delay X occurs between the reduction of concentration at intake 7 and a significant decrease in the measured concentration at exhaust 8. Should a second pulsed increase in concentration occur before the concentration at exhaust 8 decreases to its minimum, curve 2B will begin to increase and an accurate measurement of the concentration minimum is lost. Loss of accurate measurement will also occur when the concentration increases to an extremely high maximum for a short duration (spiking) and continue at a low value thereafter.
The problem of missing the maximums and minimums of a component gas concentration in a sample gas stream is particularly critical in the automobile exhaust testing area. The carburetor balance of an automobile engine is indicated by the various concentrations of carbon dioxide and oxygen in the exhaust stream. When a particular cylinder is misfiring, a radical change in the oxygen concentration in the exhaust will occur. While the component concentration measurement device is capable of sensing this radical change in concentration when it is used by itself, the combination of the measurement device with a prior art gas dryer greatly reduces sensitivity as illustrated in FIG. 2.